Aerosols in boreal forest: wintertime relations between formation events and bio-geo-chemical activity

Abstract

We studied wintertime aerosol formation and subsequent particle growth events that have been frequently observed in ambient air at a boreal forest site (SMEAR II station) in southern Finland. Aerosol size distributions, gas concentrations, biological activity and meteorological parameters have been measured at the site continuously since 1996. An important observation, based on flux measurements, was that all particle formation events were connected with either increased water evapo-transpiration or carbon dioxide uptake by the forest. However, increased H2O or CO2 fluxes did also occur on days without aerosol formation events, but these occasions were connected to cloudy or polluted days with high pre-existing aerosol concentrations. Thus, our overall conclusion is that in order for an aerosol formation event to occur, some ecological or bio-geo-chemical activity is needed. During wintertime, this activity seems to be mostly related to snow–atmosphere interactions.